1. Field of the Invention
This invention relates to calibration of material handling systems and more particularly to methods for calibrating material handling systems that fix in place workpieces such as semiconductor wafers for laser processing operations. This invention provides methods for identifying and correcting systematic errors that can be pre-computed and applied to each subsequent workpiece processed by the system.
2. Description of Related Art
Laser micro-machining is employed in a variety of operations related to semiconductor and electronics manufacturing. Some of the applications of laser processing include repair, configuration, and marking semiconductor die such as dynamic random access memory (DRAM); laser trimming of passive components; drilling vias in substrates; and impedance tuning of integrated semiconductor devices. In the example of DRAM repair, a semiconductor wafer containing DRAM integrated circuits (ICs) is tested and a list is prepared of those memory cells that fail to perform to specification. A memory yield improvement system then takes this list and directs a laser to logically remove the failed cells by focusing laser pulses onto fusible links to process them. The system then directs the laser to process fusible links associated with redundant memory cells and thereby add them to the circuit to replace the failed cells removed in the previous step. Typically laser processing of fusible links entails severing the link; however, in some cases laser processing will make an electrical connection between circuit elements. In passive component trimming, the system directs a laser onto a passive component, such as a thin film resistor, of an electronic circuit to modify its electrical properties while the circuit is being monitored by the system. In this example, the laser is directed to remove material from the resistor until the appropriate resistance value is reached. In via drilling, the system directs a laser to form holes in a substrate by either thermal effects or ablation, depending upon the laser energy, to allow electrical connections to be made between circuit layers separated vertically by insulating material. In impedance tuning, the system directs a laser at a semiconductor junction to heat it and cause re-distribution of dopants within the semiconductor junction to alter its electrical characteristics.
What these applications all have in common is that it is desirable to accurately direct a laser pulse of a specific size at an IC or a substrate. It may also be desirable to deliver at specific locations laser pulses with specific properties that include temporal pulse shapes, timing, pulse energies, and spatial energy distributions. There is considerable prior art related to these topics. For instance, U.S. Pat. Nos. 4,941,082 and 6,172,325, both assigned to the assignee of this patent application, describes methods and apparatus for accurately and rapidly delivering laser pulses to a workpiece. One thing these systems have in common is that all of them perform an alignment and calibration step prior to processing a workpiece. This step includes locating reference marks on the workpiece and using this information to position the workpiece with respect to the laser system to enable accurate processing.
It is well known that semiconductor and electronic devices are becoming denser and denser, with ever increasing amounts of circuitry being packed into smaller and smaller dimensions. As the dimensions of semiconductor and electronic components become smaller, it may be desirable to achieve higher accuracy in positioning the laser beam with respect to the workpiece. The above-mentioned U.S. Pat. Nos. 4,941,082 and 6,172,325 describe methods and apparatus for delivering a laser pulse accurately and at high speed in the X and Y coordinates, which are the coordinates generally perpendicular to the laser beam and parallel to the workpiece.
It is also well known, however, that focused laser beams have three-dimensional (3D) shapes that affect the focal spot characteristics in the Z axis, which is the axis generally parallel to the laser beam, in addition to the X and Y axes. The laser beam can be focused down to a desired spot size at a particular distance from the focus optics, but as one moves along the beam in the Z axis either closer to or farther away from the nominal focal distance, the spot size typically increases. A diagram of a typical laser beam is shown schematically in FIG. 1. The laser beam 10 is centered about an optical axis 12. The beam waist 14 is the point along optical axis 12 where beam 10 achieves its minimum spot size. The range within which laser beam 10 can maintain a spot size equal to or less than a specified diameter is referred to as depth of focus (DOF) 16. As is well known to one skilled in the art, as the minimum laser beam spot size is reduced typically by using lenses with higher numerical aperture (NA), the DOF is also reduced. It is common for there to be variations in workpiece location and surface height in all of the areas of art discussed in this patent application due to reasonable manufacturing tolerances in both the workpiece and the material handling system holding it. When the natural variations in the workpiece surface are on the same order as the DOF and desired laser beam accuracy, methods can be employed to compensate for these variations. One way to maintain a desired laser beam accuracy and spot size over the entire workpiece is to perform X, Y, and Z axis correction on the laser beam as it traverses the workpiece.
A current technique for accomplishing X, Y, and Z axis correction is represented by U.S. Pat. No. 6,483,071, in which four non-collinear points on a workpiece surface are measured and a bilinear surface is interpolated between them. The axes of a three-axis motion control system are then instructed to follow the interpolated contour in an attempt to maintain the actual workpiece surface within the DOF of the laser beam. A major shortcoming of prior art solutions is that they ignore the large amount of a priori information regarding workpiece Z-axis variations that is available for analysis. A first source is information regarding systematic variations in X, Y, and Z locations caused by the material handling system. A second source is information regarding characteristics of Z axis variations caused by particulate contamination trapped between the workpiece and the material handling system.
A desirable improvement to the state of the art would be to develop a method for performing X, Y, and Z axis correction that takes into account a priori information regarding the material handling system and the specific nature of errors caused by particulate contamination. These improvements would provide more accurate X, Y, and Z axis correction and thereby enable the use of higher NA lenses that produce a smaller laser beam spot size with shorter focal depths than those used by previous methods. The prior art also computes all of the corrections at run-time, as the workpiece is being processed. Speeding up processing, another desirable improvement, could be achieved if at least a portion of the X, Y, and Z axis correction could be pre-computed prior to processing a particular workpiece.